Recent studies have identified a small but powerful subset of IL-10-producing CD1dhiCD5+ regulatory B lymphocytes (B10) that can limit CNS inflammation and clinical signs of neurological disease in rodent models. In this multi-PI application, we hypothesize that endogenous regulatory B-cells, including the B10 subset, limit infarct size in MCAO by controlling immune-mediated inflammation triggered both in the CNS and in peripheral immune organs by focal stroke. Further, we predict that either selective induction or passive transfer of B10 cells so as to augment B10 cell frequency will provide additional regulatory effects in wild type (WT) mice, and result in smaller infarct and improved functional stroke outcome. The aims of this application are: 1) Determine the effects of B lymphocyte depletion and regulatory B10 cell restoration and treatment on infarct size, immune cellular composition and inflammatory signature in both spleen and brain hemispheres after MCAO. The hypothesis is that B cell KO mice sustain larger histological damage and poor functional outcome than do WT mice. We will determine the responsible B-cell subpopulations, including the IL-10-secreting B10 subset. 2) Evaluate the contribution of the PD-1/PD-L negative co-activation pathway to infarct size, immune cell composition and inflammatory signature in spleen and brain. The hypothesis is that activation of this receptor-ligand pathway in B lymphocytes, particularly CD1dhiCD5+ B-cells, alters outcomes from MCAO. This aim will evaluate PD-1, PD-L1 and PD-L2 expression on B-cells, confirm our preliminary findings of increased infarct size in PD-1 KO mice, extend the analysis to PD-L1 and PD-L2 KO mice, and evaluate the contribution of B10- cells to the PD-1/PD-L negative co-activation after stroke by reconstituting PD-L KO mice with WT B10-cells. 3) Evaluate the mechanisms through which B-cells inhibit microglial (MG) activation and release of neurotoxic factors. The hypothesis is that B cells, particularly CD1dhiCD5+ B regulatory cells, improve post-ischemic outcomes by suppressing microglial activation, either via secreted factors or direct cell-cell contact that enables a PD-1 negative co-activation mechanism. Specifically, this aim will evaluate interaction between PD-L+ CD19+ B-cells or IL-10-producing CD1dhiCD5+ B-reg cells and activated PD-1+ MG. The proposed studies offer high impact for the stroke field by virtue of their: 1) complete novelty, 2) focus on the inflamma-suppressive regulatory capacity of select B-cell subsets and 3) ability to lay translational groundwork for regulatory B-cell therapy in acute stroke.